1. Field of the Invention
The present invention relates to a variable speed controlling device for variable-speed-controlling an induction motor (also referred to as a motor or an induction machine), etc.
2. Description of the Related Art
A conventional induction motor driving device controls the magnetic flux and torque of a motor by controlling a voltage or an electric current on rotating coordinates in the direction of a magnetic flux axis of the motor and in the direction of a torque axis orthogonal to the magnetic flux axis. To perform such control, a speed sensor is attached to the motor, or the position of the magnetic flux of the motor is estimated and calculated from the voltage and the electric current, which are applied to the motor (refer to, for example, the description of "Sensorless Vector Control Inverter" on pp.142 to 146, Electric Society Report (DENKIGAKKAISHI) No. 2 of vol.108). That is, if a speed sensor cannot be attached to the motor, or if a cheap motor is used to which a speed sensor is not attached is used, it is general that the position of the magnetic flux of the motor is estimated and calculated from a voltage or an electric current, like in the latter case.
FIG. 1 is a block diagram exemplifying the conventional system.
In this figure, 1 indicates an electric power converting circuit; 2 indicates an induction motor (IM); 6 indicates a voltage command value calculating circuit; 8 and 11 indicate coordinate transforming circuits; 12 indicates an integrator; and 21 indicates an electric current detector. The position of magnetic flux .theta.* is obtained from a primary angular frequency command value .omega..sub.1 * in the integrator 12, and the coordinate transforming circuit 11 transforms the electric current value detected by the electric current detector 21 into electric current values i.sub.M and i.sub.T on the rotating coordinates by using the position of the magnetic flux .theta.*. The voltage command value calculating circuit 6 obtains voltage command values v.sub.M * and v.sub.T * from the output of the coordinate transforming circuit 11 and the primary angular frequency command value .omega..sub.1 *. The coordinate transforming circuit 8 transforms the outputs v.sub.M * and v.sub.T * from the voltage command value calculating circuit 6 into a voltage command v.sub.1 *, and controls the electric power converting circuit 1 using the voltage command v.sub.1 * for driving the induction motor 2.
In a driving device for an induction motor which does not have a speed sensor attached, however, an error may sometimes occur in the estimation of the position of the magnetic flux, which causes a deviation of an operating point. Especially, this tendency is significant in an operating area in which a primary frequency is low.
The object of the present invention is therefore to implement superior control by obtaining an error signal which varies depending on a deviation of an operating point to be obtained and correcting the deviation of the operating point based on the error signal.